1. Field of Invention
This invention relates to a package for complex semiconductor devices and a ceramic terminal block useful for the package. Here, the complex semiconductor device means that more than one device is accommodated into one package, and more particularly, means one of the semiconductor devices has already been provided with its own leads as connecting terminals. This invention can be applied to many kinds of complex semiconductor devices, but a semiconductor package accommodating both a Peltier device and a semiconductor laser will be explained as an example of the present invention.
2. Description of Related Art
This application claims the priority with respect to Japanese Patent Application No. 274228/1996 filed Sep. 24, 1996, the subject matter of which is incorporated by reference, as if fully set forth herein.
Since semiconductor lasers (LDs) and light emitting diodes (LEDs) inherently emit light by supplying current, they are strongly running hot. An excessive rise of the temperature of the light emitting devices brings about a decrement of emission efficiency. Therefore, the devices must be cooled as to prevent the reduction of efficiency or as to generate light of great power.
A heat sink is often effective only for cooling the heated devices, but is sometimes insufficient to cool overheated devices, because the cooling capacity of the heat sink depends on natural radiating. Further, the performance of devices must often be stabilized, by keeping the devices at a constant temperature. Furthermore, even in the case of photodetectors, for example, an avalanche photodiode, dissipating much heat, requires a certain cooling mechanism. Strict temperature control is needed for a sophisticated photodiode that has a light receiving layer with an MQW (Multi-Quantum Well) structure, by utilizing the transition between side modes and so on for obtaining a high sensitive wavelength selectivity.
It may be considered that the whole of devices would be put into a thermostatic bath in order to control the temperature or to cool the devices. This method is, however, unrealizable, because it would decrease the freedom of designing optical systems and it would significantly increase the cost of production.
As a practical matter, it is possible to provide a package structure into which a Peltier device can be placed, and to place a laser, a photodiode, a light receiving device, and various circuit devices on the Peltier device so that they can be thermally controlled. A Peltier device, being a type of semiconductor device, is small enough to be accommodated in a package with other semiconductor devices. A Peltier device is made by layering a metal, a p-type semiconductor, a metal, an n-type semiconductor and a metal in order. The temperature of the intermediate metal layers can be varied by changing the direction of current flowing through the device. When a current flows in one direction, an intermediate electrode radiates heat, and when a current flows in the reverse direction, the intermediate electrode absorbs heat.
The adjustments of the current direction and the current quantity make it possible to control the temperature of the intermediate electrode of the Peltier device. It is, in general, possible to heat and cool an object device by mounting it onto an intermediate electrode with an insulator and supplying a current to the Peltier device. Further, the object device can be maintained at a predetermined temperature by measuring its temperature using a chip-thermistor and making appropriate adjustments to the current flowing through the Peltier device.
Such a complex semiconductor device accommodates a Peltier device and a semiconductor laser together in a package. Several complex semiconductor devices have been proposed, which will be explained briefly as follows:
(1) JAPANESE LAID OPEN UTILITY MODEL NO.3-39867 (39867/1991) PA0 TITLE OF THE UTILITY MODEL: "SEMICONDUCTOR LASER MODULE" PA0 (2) JAPANESE LAID OPEN UTILITY MODEL NO.5-15440 (15440/1993) PA0 TITLE OF THE UTILITY MODEL: "PACKAGE FOR ACCOMMODATING AN OPTICAL SEMICONDUCTOR DEVICE" PA0 (3) JAPANESE LAID OPEN PATENT NO.5-67844 (67844/1993) PA0 TITLE OF THE INVENTION: "SEMICONDUCTOR LASER MODULE" PA0 (4) JAPANESE LAID OPEN PATENT NO.8-37247 (37247/1996) PA0 TITLE OF THE INVENTION: "PACKAGE FOR ACCOMMODATING OPTICAL SEMICONDUCTOR DEVICES"
The main object of this utility model laser module is to control the temperature of a semiconductor laser by a Peltier device put into the package. Electrode pads of the semiconductor laser are connected to package electrode patterns with ribbon-type leads.
This utility model puts a Peltier device on the bottom board of the package, deposits an insulator on the Peltier device, and puts a semiconductor laser on the insulator.
This invention proposes a package having a bottom board being made of AlN (aluminum nitride), a plurality of Peltier units attached to the bottom board, a semiconductor laser, a light receiving device and so on placed on the Peltier devices. The semiconductor laser is positioned on a heat sink put on the Peltier devices. However, in this structural arrangement including a Peltier device in a package, there is a difficult problem of an electric contact between the Peltier device and the package. The above cited prior art documents (1) to (3) mention nothing about the contact between electrode patterns of a package and leads of a Peltier device. There are no suggestions for arrangements to provide electrical contacts.
The electrical wiring in a package is done by bonding conductive pattern electrodes on a ceramic terminal block to electrode pads formed on a light emitting device, a photodetector and ICs of e.g., an amplifier circuit, a logic circuit and so on, with wires of gold (Au) and so on. This requires that the electrode pads on the devices and the patterned electrodes on the ceramic terminal should be arranged approximately at the same level, that is, the difference in height between the terminal electrode patterns and the electrode pads must be small. This requirement is based on the small length of a needle part of a bonding tool.
Generally a Peltier device is put at the intermediate level between the package bottom surface and the devices, that is, a laser diode, a photodetector, an amplifier circuit, a logic circuit and so on that radiate heat. There is a difference in height between the Peltier device electrodes and pattern electrodes on the ceramic terminal block. The wirebonding of, for example, gold(Au) therebetween is so difficult that the Peltier unit is usually accommodated in its own package having lead pins. Then the Peltier device is independently used unlike other devices.
The leads of the Peltier device are soldered to the pattern electrodes on the terminal block so as to establish electric conduction between the package and the Peltier device.
There are several problems in the step of soldering leads of the Peltier device with conducting parts (electrode patterns) of the ceramic terminal block. Heating the whole of the package is desirable from the viewpoint of soldering, but the Peltier device that is an thermoelectric cooling device is deficient in thermal stability. When an electron device like a Peltier device having such a poor thermal stability were entirely heated, the electron device would be destroyed by the heat. Hence, the lead-electrode pattern soldering by heating all is impossible.
At the present time, lead pins of the Peltier device and metallized electrode patterns are soldered by the processes of bending ends for the lead pin at an adequate angle, putting the lead pins on electrode patterns, heating both the pins and the electrode patterns, and soldering the lead pins on the metallized electrode patterns.
Soldering by hand work has disadvantages. For example, hand soldering takes much time. As mentioned above, the leads of the Peltier device should be bent, should be brought into contact with the wire, and should be heated by attaching a soldering iron, as pressing the lead with a pair of cutting pliers. Further, a solder should be supplied to the heated lead, be melted, and be extended on the metallized wire. After the solder had been solidified, a tip of the cutting pliers should be removed from the lead. There are many metallized electrode patterns in a package and the area of an electrode pattern for soldering a lead is very narrow. The electrode patterns are flat and there is nothing to hold the leads temporarily, so that cutting pliers or tweezers are necessary for keeping the leads whenever the leads are soldered to the electrode pattern. This hand work requires much time and high skill. At the present, it takes several minutes to solder a lead pin with a metallized electrode (pattern).
When hard and thick leads are connected to electrode patterns by soldering, faulty soldering is apt to occur. Even when the soldering seems to be perfect, there sometimes occurs a faulty soldering connection. In this case, it is impossible to find out the faulty condition without conducting a test. Furthermore, it is difficult to distinguish the electrode patterns to be connected with the Peltier device leads from a number of other metallized electrode patterns by mere eye observation. There exists only numbers for designating the wires to be connected with the leads of the Peltier device. Therefore, there is a possibility of connecting the Peltier device leads to wrong electrode patterns by soldering.
These drawbacks result from the fact that thin metallized electrode patterns of a package are not suitable for the connection with a hard thick material like a lead. Originally, the metallized electrode patterns are not for soldering of thick leads but for wirebonding of thin wires.
FIG. 1 shows a part of a terminal structure on a ceramic block of a prior package including some improvements which was proposed by:
FIG. 1 (Prior Art) is an explanatory figure for showing a part of the ceramic terminal block being a part of the package. In general, a metallized electrode patterns 1 are printed on a ceramic block 2, and a part of an upper surface 6 of the ceramic terminal block 2 is pressed by a ceramic block 3. The upper surface 6 of the terminal block 2 is level, and a front end surface 4 of the ceramic terminal block 2 is flat.
A semicylindrical groove 5 is bored on the front end surface 4 of the ceramic block 2 in order to obtain an easy positioning of a lead 8 of a Peltier device. The lead 8 of the Peltier device is fitted vertically to the semicylindrical groove 5, and is connected to the metallized wire 1 by a solder 7. The other electrode patterns have no grooves, so that it is easy to distinguish the electrode patterns for the leads 8 from a lot of other electrode patterns for wirebonding.
FIG. 2 (Prior Art) shows a state where the lead 8 of the Peltier device is connected to the predetermined electrode pattern 1 by the solder 7. As explained above, the groove 5 prevents the connection between the lead 8 and the electrode pattern 1 from making a mistake, and gives rise to a simplified soldering of the lead on the electrode pattern without pressing the lead with cut pliers or tweezers.
Metallized electrodes printed on a terminal block should be ordinarily connected to pads of IC chips having no leads by wirebonding. The head capacity is not appropriate for soldering. The Peltier device lead should be bent for connecting the lead to the electrode pattern. An unsuitable bend on the tip results in default of connection between the lead and the electrode pattern.
An operator must press the lead with a soldering iron, holding the lead with pliers or tweezers for forcing the lead to the electrode pattern. The metallized electrode patterns are thin, but their practical thermal capacity is comparatively large. The electrode pattern must be heated by contacting a tip of the soldering iron till the electrode pattern is heated up to a temperature more than the melting point of the solder. Since ceramic has high thermal conductivity, heat escapes from the electrode pattern. Hence, it is difficult to raise the temperature of the electrode pattern. It takes a certain time on heating the wire part up to more than the melting point of the solder, which is a cause of taking much time to solder the Peltier device lead to the electrode pattern. Further, a high degree of proficiency is required for soldering.
The prior document (4) (37247/96) would not commit an error in connecting the lead of the Peltier device to the electrode pattern due to the longitudinal groove drilled in the side surface of the metallized part. Since the position of the lead is predetermined by the groove, soldering is easily carried out. However, the arrangement shown in document (4) does not solve the problem of direct-connection between the lead and the electrode pattern. It takes much time to heat the electrode pattern having a large practical heat capacity by a soldering iron. As shown in FIG. 2, since the lead is connected at right angles to the electrode pattern by soldering, the soldered area is so narrow that the connection power of soldering becomes weak. The solder connection is likely to break down by the cycle of temperature changes and some vibrations.